1. Field
The present invention relates to a lithographic apparatus including a chuck constructed to hold a substrate table, to a stage apparatus for positioning an object and to a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus, the patterning device may be held by a support. Commonly, the apparatus will be provided with a positioning device (comprising e.g one or more linear motors or actuators) to move the support, thereby moving the patterning device held by the support. In the lithographic apparatus, the substrate is, in general, supported by a substrate table, the substrate table being mounted to a so-called chuck or carrier by means of clamping. The substrate table may e.g. be clamped to the chuck by means of a vacuum clamp or electrostatic clamp, although mechanical ways of clamping may also be considered feasible. The chuck may also be referred to as the mirror block in case the chuck is provided with one or more reflective surfaces that are used as a target for an interferometer beam. The chuck is in general displaceable using a similar positioning device as is used to position the support, i.e. a positioning device comprising one or more electromagnetic motors or actuators. In a lithographic apparatus as known in the art, the substrate table is provided with a clamping mechanism such as a vacuum clamp or an electrostatic clamp to clamp the substrate and hold the substrate during the exposure process. Once the exposure of the substrate is performed, the substrate can be removed from the table and replaced by a next substrate to be exposed. Similar provisions are in general available for clamping the patterning device to the support.
During the exposure process, it is important to know the actual position of the substrate relative to the patterning device in order to ensure that the pattern is projected on the correct target area of the substrate. In order to achieve this, the lithographic apparatus is provided with one or more measurement systems for determining the position of the substrate relative to the patterning device. In general, the relative position of the substrate with respect to the patterning device is determined using an alignment procedure. In such a procedure, the position of the patterning device relative to the substrate can e.g. be determined in an optical way, e.g. by any suitable operation in the projection system to have a pattern projected onto an appropriate part of the surface of the substrate. When the position of the support and the chuck are known in the aligned position of the patterning device and the substrate, the relative position of the substrate and the patterning device can be determined based on a position measurement of the support and the chuck. As such, controlling the position of the chuck and support can then be used to maintain the patterning device and the substrate at a required position with respect to each other or to control the relative position of the substrate and patterning device. To achieve this, the lithographic apparatus is, in general, equipped with a control unit arranged to control the positioning devices that displace the support and the chuck. This control unit can e.g. be arranged in such manner that both the substrate and the patterning device can follow a predetermined set-point.
In such a set-up, the accuracy of the exposure process, i.e. the accuracy with which the pattern is projected on the required position on the substrate relies e.g. on the position of the chuck relative to the substrate or substrate table being known. In principle, in case both the chuck and the substrate table would be infinitely stiff and clamped to each other such that no relative displacement can occur, the relative position between the chuck and table would remain constant over time. In such a situation, the position of the substrate can be determined on the basis of a position measurement of the chuck.
In practice, it has been found that the chucks as currently applied in lithographic apparatuses may deform due to a.o. the acceleration forces exerted on it, as the stiffness of such chucks is not infinitely high. It has further been found that this deformation to some extent remains present after the acceleration phase. As a result, such deformation of the chuck may contribute to an inaccurate exposure, i.e. the pattern of the patterning device may be exposed on the wrong place on the substrate, because the deformation may affect the position of the chuck relative to the substrate table.
It is therefore noted that a deformation of a chuck is undesired, as this would adversely affect an accuracy of a positioning of the pattern on the substrate, thereby deteriorating a performance of the lithographic apparatus.